When we need to measure the capacity of a container to determine how many milliliters or liters are in the container, we use measuring cylinders or jugs. These cylinders or jugs have markings that show the measurement. You will notice that when you want to take a measurement of liquid, the level of the liquid does not form a straight line, but a line that is curved at the edges.
You need to:
- stand level with the line
- ignore the curved edges
- squarely look at the level
- take the measurement where the straight line forms
- Thermometers
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A thermometer is a device that measures temperature or temperature gradient using a variety of different principles. Traditional thermometers work by using a material inside a glass tube that changes its form when either heated or cooled, typically mercury or a cooled alcohol, either ethanol or toluene. The liquid expands and travels up the tube as it is heated, and contracts and moves down the tube when it is cooled. Thermometers are calibrated in standard temperature units called Fahrenheit or Celsius and the scale used depends on your global location, although some types of thermometers offer readings in both scales. Modern types of thermometers are also available that do not use mercury or alcohol and provide a digital display. Until relatively recently, the most commonly available thermometer was the mercury glass thermometer. These were most used in the medical profession to take a persons temperature. But now the market is full of different types of thermometers to cover your every need.
Note
The point, at which the colored liquid or mercury ends, is where the measurement is taken.
- Scales and Balances
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A weighing scale is a measuring instrument for determining the weight or mass of an object. A spring scale measures weight by the distance a spring deflects under its load. A balance compares the torque on the arm due to the sample weight to the torque on the arm due to a standard reference weight using a horizontal lever. Balances are different from scales, in that a balance measures mass (or more specifically gravitational mass), where as a scale measures weight (or more specifically, either the tension or compression force of constraint provided by the scale. The simplest and oldest weighing mechanism is the equal-arm balance. It has an equal-arm balance which is a bar with two pans hanging from each end and a support at the center of the bar. The bar balances on this center support. To use an equal-arm balance, an object of unknown weight is placed in one of the pans, and objects of known weight are added to the other pan until the bar holding he pans is balanced; then the weight of the unknown load is the same as the known weight in the other pan. A spring scale has a platform that is linked to a spring. The spring either extends or squeezes together to balance the load on the platform. A needle indicates the weight of the load. Many bathroom scales are spring scales. In your job, you will come across electronic scales. They are modern versions of scales and are much faster and more accurate than the mechanical scales. Electronic scales can also be linked to computer systems.
Balance scales allow to directly compare gravitational mass within a gravitational field. Such devices have been in use since at least the Middle Bronze Age (shown is a balance for weighing tobacco dating to the mid-19th century).
Balancing scale
1.2.5 Watches and Clocks
We can measure time on either an analogue or digital clock. An analogue clock is marked in 12 hour intervals and the clock hands need to pass the12 for the second time to indicate a 24 hour period.
To indicate the period between 12 o’clock midnightand12 o’clock midday, we use the a.m. abbreviation i.e.5:00 am. To indicate the period between 12 o’clock noon and 12 o’ clock midnight, we use the p.m.
abbreviation i.e. 5:00 p.m. A digital clock works on a 24 hour basis. We do not use a.m or p.m to indicate the Time of day, because 5:00 a.m is 05h00 And5:00 p.m is 17h00 1
The three figures will help us to read a watch or a clock.
Hour hand indicates hours and Minute-hand indicates minutes.
In figure (i) hour-hand is indicating number 1 or just after 1. So, we read it as 1 hour. The minute hand indicates 4 divisions after the number 3, i.e., 3 x 5 + 4 = 19, or 19 divisions from 12. So we read it as 19 minutes.The watch indicates that the time is 1 hour 19 minutes.
We may express it as 1: 19. We may say it is 19 minutes past 1.At or after the number that the hour-hand indicates, the number expresses the hour.
At or after the number that the minute-hand indicates, the number multiplied by 5 and number of divisions after the number, express the minutes. If minute hand indicates 3 divisions after number 7, then minutes is 7 x 5 + 3 = 38 minutes.In figure (ii) hour hand is between 3 and 4. So, we say it is 3 hours. The minute hand is 2 divisions after number 8. So, minute is 8 x 5 + 2 = 42 minutes.
The time is 3 hours 42 minutes. We may write it as 3 : 42. We may say it is 42 minutes past 3,
or, (60 – 42 = 18 minutes) 18 minutes to 4.In figure (iii) expresses 8: 35 or 35 minutes past 8 or, (60 – 35 = 25 minutes) 25 minutes to 9.
1.2.6 Vernier Caliper
The vernier caliper is used to make semi- accurate measurements for inside, outside and depth dimensions. Standard vernier callipers are available in sizes 150mm to 250mm. Custom- made vernier callipers can be made to specifications if required. Graduations, (that determine the accuracy of the instrument) are usually 0,02mm or 0,05mm on the vernier scale. A calliper can be as simple as a compass with inward or outward-facing points. The tips of the calliper are adjusted to fit across the points to be measured, the calliper is then removed and the distance read by measuring between the tips with a measuring tool, such as a ruler. It is used in many fields such as mechanical engineering, metalworking, woodworking, science and medicine. The vernier, dial, and digital callipers give a direct reading of the distance measured to high accuracy. They are functionally identical, with different ways of reading the result. These callipers comprise a calibrated scale with a fixed jaw, and another jaw, with a pointer, that slides along the scale. The distance between the jaws is then read in different ways for the three types. The simplest method is to read the position of the pointer directly on the scale. When the pointer is between two markings, the user can mentally interpolate to improve the precision of the reading. This would be a simple calibrated calliper; but the addition of a vernier scale allows more accurate interpolation, and is the universal practice; this is the vernier calliper. Vernier, dial, and digital callipers can measure internal dimensions (using the uppermost jaws in the picture at right), external dimensions using the pictured lower jaws, and in many cases depth by the use of a probe that is attached to the movable head and slides along the centre of the body. This probe is slender and can get into deep grooves that may prove difficult for other measuring tools. The vernier scales may include metric measurements on the lower part of the scale and inch measurements on the upper, or vice versa, in countries that use inches. Vernier callipers commonly used in industry provide a precision to 0.01 mm ( micrometres), or one thousandth of an inch. They are available in sizes that can measure up to 1,829 mm (72 in).
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Parts of a vernier caliper:
- Outside jaws: used to measure external diameter or width of an object
- Inside jaws: used to measure internal diameter of an object
- Depth probe: used to measure depths of an object or a hole
- Main scale: scale marked every mm
- Main scale: scale marked in inches and fractions
- Vernier scale gives interpolated measurements to 0.1 mm or better
- Vernier scale gives interpolated measurements in fractions of an inch
- Retainer: used to block movable part to allow the easy transferring of a measurement
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1.2.7 Micrometer screws
1.2.7 Micrometer screws
A micrometer screw gauge, is a device incorporating a calibrated screw used widely for precise measurement of small distances in mechanical engineering and machining as well as most mechanical trades, along with other metrological instruments such as dial, vernier, and callipers. They are also used to measure diameter of holes. Micrometers are often, but not always, in the form of callipers.
Basic types
The three most common types of micrometer are;
- Outside micrometer (aka micrometer caliper), typically used to measure wires, spheres, shafts and blocks.
- Inside micrometer, used to measure the diameter of holes.
- Depth micrometer, measures depths of slots and steps.
Note The names are based on their application
Micrometers use the principle of a screw to amplify small distances (that are too small to measure directly) into large rotations of the screw that are big enough to read from a scale. The accuracy of a micrometer derives from the accuracy of the thread-form that is at its heart. The basic operating principles of a micrometer are as follows:
- The amount of rotation of an accurately made screw can be directly and precisely correlated to a certain amount of axial movement (and vice versa), through the constant known as the screw’s lead (/ˈliːd/). A screw’s lead is the distance it moves forward axially with one complete turn (360°). (In most threads [that is, in all single-start threads], lead and pitch refer to essentially the same concept.)
- With an appropriate lead and major diameter of the screw, a given amount of axial movement will be amplified in the resulting circumferential movement.
For example, if the lead of a screw is 1 mm, but the major diameter (here, outer diameter) is 10 mm, then the circumference of the screw is 10π, or about 31.4 mm. Therefore, an axial movement of 1 mm is amplified (magnified) to a circumferential movement of 31.4 mm. This amplification allows a small difference in the sizes of two similar measured objects to correlate to a larger difference in the position of a micrometer’s thimble.
In classic-style analogy micrometers, the position of the thimble is read directly from scale markings on the thimble and shaft. A vernier scale is often included, which allows the position to be read to a fraction of the smallest scale mark. In digital micrometers, an electronic readout displays the length digitally on an LCD display on the instrument. There also exist mechanical-digit versions, like the style of car odometers where the numbers “roll over”.
1.2.8 Stopwatch
A stop watch is a handheld timepiece designed to measure the amount of time elapsed from a particular time when activated to when the piece is deactivated. A large digital version of a stopwatch designed for viewing at a distance, as in a sports stadium, is called a stopcock. The timing functions are traditionally controlled by two buttons on the case. Pressing the top button starts the timer running, and pressing the button a second time stops it, leaving the elapsed time displayed. A press of the second button then resets the stopwatch to zero. The second button is also used to record split times or lap times. When the split time button is pressed while the watch is running, the display freezes, allowing the elapsed time to that point to be read, but the watch mechanism continues running to record total elapsed time. Pressing the split button a second time allows the watch to resume display of total time.Mechanical stopwatches are powered by a mainspring, which must be periodically wound up by turning the knurled knob at the top of the watch.
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1.2.9 Chemical Balances
A beam balance of great precision used in quantitative chemical analysis.
1.2.10 Electric Meters
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Electric meters are instruments used to measure electrical values. The two most common meters are the ammeter (which measures ampere) and the voltmeter (which measures volts).The ammeter and voltmeter are variations of the galvanometer. Inside a galvanometer is a small magnet that sets up a magnetic field. This magnetic field generates a force that we can measure when current flows through a coil.
Below, the meter mechanisms are shown schematically.
The ammeter diverts electricity through a coil via a shunt (illustrated beneath the device), measuring the amount of current flowing through the circuit in amperes. It is connected in series, or directly in the line of the circuit. The voltmeter is connected in parallel, so that the light bulb falls between its two connections into the circuit. It is designed to measure potential differences. To ensure that it removes a minimum of current from the circuit, the voltmeter’s resistance (shown by the jagged black line beneath the device
Exercise
1. Draw straight lines in your workbook that are of the following lengths:
a) 150mm
b) 0,025m
c) 8,75cm
d) 0,00019km
e) 4m
2. Technician A says that 1 liter equals 1000 cubic centimeters. Technician B says the most versatile precision measuring instrument for an automotive technician is the vernier caliper.
Who is right?
a. Technician A b. Technician B c. Both A and B d. Neither A nor B
3. Technician A says that one revolution of the micrometer thimble equals 1⁄₄₀ inch. Technician
B says that one revolution of the micrometer thimble equals 0.025 inch. Who is right?
a. Technician A b. Technician B
c. Both A and B d. Neither A nor B
4. Technician A says that the English system of measurement is based on the number 10.
Technician B says that calipers, dividers, and telescopic gauges are precision measuring instruments. Who is right?
a. Technician A b. Technician B
c. Both A and B d. Neither A nor B
1.3 SI AND IMPERIAL MEASUREMENTS
The System International (S.I) has common units to measure quantities that make communicating measurement between countries easy. The most important measurement concepts are discussed further. The International System of Units was established in 1960 and is commonly referred to as the SI units .The SI unit system is based on the metric system, which means that we use 10 (and Its subsequent multiples) as the base number to increment various measures. For example
1 meter = 10 decimeters = 100 centimeters = 1000 millimeters
1.3.1 Distance-Length
In geometric measurements, length most commonly refers to the longest dimension of an object. In certain contexts, the term “length” is the distance between two points i.e.
X y
In the physical sciences and engineering, when one speaks of “units of length”, the word “length” is synonymous with “distance”. There are several units that are used to measure length. Units of length may be based on lengths of human body parts, the distance travelled in a number of paces, the distance between landmarks or places on the Earth, or arbitrarily on the length of some fixed object.
In the International System of Units (SI), the basic unit of length is the meter (m) and is now defined in terms of the speed of light. The centimetre (cm) and the kilometre, (km) derived from the meter, are also commonly used units. In U.S. customary units, English or Imperial system of units, commonly used units of length are the inch, the foot, the yard, and the mile.
- Scales and Balances
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